As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research on secondary batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, exhibiting high energy density, discharge voltage, and output stability, is very high.
In addition, secondary batteries may be classified based on the structure of an electrode assembly having a cathode/separator/anode structure. For example, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which a long sheet type cathode and a long sheet type anode are wound while a separator is disposed between the cathode and the anode, a stacked type structure in which pluralities of cathodes and anodes having a predetermined size are sequentially stacked while separators are disposed respectively between the cathodes and the anodes, or a stacked/folded type structure in which pluralities of cathodes and anodes having a predetermined size are sequentially stacked while separators are disposed respectively between the cathodes and the anodes to constitute a bi-cell or a full-cell and then a plurality of bi-cells or full-cells is folded.
Recently, much interest has been taken in the increase in area of a battery case and the decrease in thickness of the battery case according to the increase in capacity of a battery. As a result, the use of a pouch-shaped battery configured to have a structure in which such a stacked or stacked/folded type electrode assembly is mounted in a pouch-shaped battery case made of an aluminum laminate sheet has gradually increased because of low manufacturing costs, light weight, easy modification in shape, etc.
However, the above-mentioned conventional electrode assemblies has the following several problems.
First, the jelly-roll type electrode assembly is prepared by winding the long sheet type cathode and the long sheet type anode in a dense state such that the jelly-roll type electrode assembly has a circular or oval structure in section. As a result, stress, caused by expansion and contraction of the electrodes during charge and discharge of a battery, may accumulate in the electrode assembly and, when an accumulation level of the stress exceeds a specific limit, the electrode assembly may be deformed. The deformation of the electrode assembly results in non-uniformity of a gap between the electrodes. As a result, the performance of the battery may be abruptly deteriorated and the safety of the battery may not be secured due to an internal short circuit of the battery. In addition, it is difficult to rapidly wind the long sheet type cathode and the long sheet type anode while uniformly maintaining the gap between the cathode and anode with the result that productivity is lowered.
Secondly, the stacked type electrode assembly is prepared by sequentially stacking the plurality of unit cathodes and the plurality of unit anodes. For this reason, it is necessary to additionally perform a process for transferring electrode plates which are used to prepare the unit cathodes and the unit anodes. In addition, much time and effort are required to perform the sequential stacking process with the result that productivity is lowered.
In order to solve the above-mentioned problems, there has been developed a stacked/folded type electrode assembly having an improved structure, which is a combination of the jelly-roll type electrode assembly and the stacked type electrode assembly. The stacked/folded type electrode assembly is configured to have a structure in which pluralities of cathodes and anodes having a predetermined size are stacked in a state in which separators are disposed respectively between the cathodes and the anodes so as to constitute a bi-cell or a full-cell and then a plurality of bi-cells or a plurality of full-cells is folded using a long separator sheet. The details of the stacked/folded type electrode assembly are disclosed in Korean Patent Application Publication No. 2001-0082058, No. 2001-0082059, and No. 2001-0082060, which have been filed in the name of the applicant of the present patent application.
FIG. 1 is a series of views typically showing an exemplary process for preparing a conventional stacked/folded type electrode assembly and FIG. 2 is a graph typically showing the change in thickness and capacity of the conventional stacked/folded type electrode assembly due to the increase in number of stacks.
Referring to these drawings, the stacked/folded type electrode assembly is prepared, for example, by arranging bi-cells 10, 11, 12, 13, and 14 on a long separator sheet 20 and sequentially folding the bi-cells 10, 11, 12, 13 and 14 from one end 21 of the separator sheet 20.
As the number of stacked electrode plates is increased, the thickness of a bi-cell is increased with the result that the capacity of the bi-cell is also increased.
The stacked/folded type electrode assembly prepared using the above method solves the problems caused in the above-mentioned jelly-roll type and stacked type electrode assemblies. In a bi-cell, however, the number of stacked electrode plates is odd. Furthermore, as shown in FIG. 2, a section in which the total capacity of the bi-cell is not increased in proportion to the increase in number of the stacked electrode plates is generated with the result that energy density proportional to the thickness of the battery cell is not satisfied.
In addition, as can be seen from Table 1 below, the number of electrode tabs is increased in proportion to the increase in number of the electrode plates. It is necessary to weld the electrode tabs in a bundle during a manufacturing process of the battery cell with the result that welding processability and efficiency of the electrode tabs are greatly lowered.
TABLE 1Conventional stacks and foldsNumber of cathodes =Number of anodes =Number of cathode tabsNumber of anode tabs3 stacks455 stacks787 stacks10119 stacks1314. . .N stacks(3N − 1)/2(3N + 1)/2
Therefore, there is a high necessity for technology that is capable of fundamentally solving the above problems.